1. Field of the Invention
The present invention relates to operational amplifiers. More specifically, the invention relates to an operational amplifier in which one stage of voltage amplification circuitry is provided.
2. Description of the Related Art
An operational amplifier formed of a connection of a plurality of stages (for example, two stages) of phase inversion type voltage amplification circuits is conventionally known. The operational amplifier formed of such a connected plurality of stages of voltage amplification circuits however suffers from turning points (poles) in the frequency characteristic corresponding to the number of the stages, and a large phase delay as a whole results due to a phase delay in each stage, thereby deteriorating the frequency characteristic. In order to avoid such disadvantages, devices having one stage of voltage amplification circuits and allowing voltage amplification without phase inversion have been proposed.
FIG. 1 is an electric circuit diagram showing such an operational amplifier having one stage of voltage amplification circuits. Referring to FIG. 1, the operational amplifier includes in the upper and lower parts of the circuit diagram, a differential amplification circuit 1a formed of a differential connection of NPN transistors Q.sub.1 and Q.sub.2, and a differential amplification circuit lb formed of a differential connection of PNP transistors Q.sub.4 and Q.sub.5, respectively. A power supply voltage Vcc is supplied to the collector of transistor Q.sub.1, the collector of transistor Q.sub.2 is connected to a constant current circuit 5a, and the power supply voltage Vcc is supplied to constant current circuit 5a. The emitters of transistors Q.sub.1 and Q.sub.2 are both connected to constant current circuit 4b, and constant current circuit 4b is supplied with a power supply voltage -Vcc. The power supply voltage -Vcc is supplied to the collector of PNP transistor Q.sub.4, the collector of PNP transistor Q.sub.5 is connected to constant current circuit 5b, and constant current circuit 5b is supplied with the power supply voltage -Vcc. The emitters of transistors Q.sub.4 and Q.sub.5 are both connected to a constant current circuit 4a, and constant current circuit 4a is supplied with the power supply voltage Vcc. The bases of transistors Q.sub.1 and Q.sub.4 are connected to an input terminal IN.sub.1, and the bases of transistors Q.sub.2 and Q.sub.5 are connected to an input terminal IN.sub.2.
The collector of transistor Q.sub.2 is connected to the emitter of transistor Q.sub.21 with its base grounded, and the collector of transistor Q.sub.5 is connected to the emitter of a transistor Q.sub.22 which has also its base grounded. A power supply 6a is connected to the base of transistor Q.sub.21, and a power supply 6b is connected to the base of transistor Q.sub.22. A buffer circuit 3 for current amplification is connected to the connection point of the collectors of transistors Q.sub.21 and Q.sub.22. Z.sub.L is a load impedance for transistors Q.sub.21 and Q.sub.22 having their bases grounded and is actually an impedance inside the buffer circuit 3 in an actual circuit.
In the operational amplifier shown in FIG. 1, the current changes of differential amplification circuits 1a and 1b due to signals applied to input terminals IN.sub.1 and IN.sub.2 are extracted from the collectors of base-grounded transistors Q.sub.21 and Q.sub.22, the difference between the currents flowing through these transistors Q.sub.21 and Q.sub.22 is converted into a voltage value by the load impedance Z.sub.L, and the voltage value is output from an output terminal OUT.
The operational amplifier shown in FIG. 1 has advantages over the one formed of a plurality of stages of voltage amplification circuits in that it has a relatively simple circuit configuration, a higher gain band width product ft, and an excellent high frequency characteristic due to its reduced phase delay.
However, in the conventional operational amplifier shown in FIG. 1, the transistor Q.sub.2 of differential amplification circuit 1a and transistor Q.sub.5 of differential amplification circuit 1b, and base-grounded transistors Q.sub.21 and Q.sub.22 are connected together as loads to constant current circuits 5a and 5b in the upper and lower parts, respectively. The current of constant current circuit 5a flow through transistors Q.sub.2 and Q.sub.21. The current of constant current circuit 5b flow through transistors Q.sub.5 and Q.sub.22. The characteristic changes of constant current circuits 5a and 5b adversely affect the operations of transistors Q.sub.21 and Q.sub.22 having their bases grounded. More specifically, if the characteristics of the constant current circuits 5a and 5b in the upper and lower parts are not uniform at the time of manufacture, the balance of the current to be shunted, and thereby to flow through transistors Q.sub.21 and Q.sub.22, changes between the upper and lower parts, thus producing a difference between the operation currents of transistors Q.sub.21 and Q.sub.22. As a result, an unnecessary D.C. component is output as an offset voltage to output terminal OUT at the final stage.
Furthermore, if the power supply voltage Vcc changes, or the temperature changes, the currents of constant current circuits 5a and 5b change, thus changing the operation currents of base-grounded transistors Q.sub.21 and Q.sub.22, and, therefore, the frequency characteristics of transistors Q.sub.21 and Q.sub.22 also change.